1. Field of the Invention
This invention relates generally to ground proximity warning systems, and more particularly to a ground proximity warning system for aircraft of the type that provides warnings including warnings of excessive terrain closure rate and of excessive descent below a glide slope beam wherein the criteria required to generate an excessive terrain closure rate warning are modified when the aircraft is on a glide slope beam to minimize nuisance warnings.
2. Description of the Prior Art
Ground proximity systems, including ground proximity warning systems that provide warnings when the terrain closure rate of an aircraft is excessive and when the aircraft is below a glide slope beam are known. Typical systems that provide a warning when the terrain closure rate is excessive are disclosed in U.S. Pat. Nos. 3,715,718, 3,934,221, 3,934,222, 3,944,968 and 3,958,219. Examples of systems that are responsive to a glide slope radio beam for generating a warning when the aircraft descent excessively below the glide slope are described in U.S. Pat. Nos. 3,925,751 and 3,947,809. An example of a system that provides warnings of various hazardous flight conditions including excessive closure rate and excessive descent below the glide slope is disclosed in U.S. Pat. No. 3,946,358. All of the above-mentioned patents are assigned to the same assignee as the assignee of the present invention, and incorporated herein by reference.
While the devices described in the above-mentioned references do provide satisfactory performance in most situations, there is still a tendency for the excessive terrain closure warning systems to generate false, or nuisance, warnings under certain circumstances, such as on normal ILS approaches to certain airports that are surrounded by mountains, such as, for example, airports at St. Johns, New Foundland, Zurich, Switzerland, Lisbon, Portugal and Madrid, Spain.
Although much effort has been expended in attempting to solve the aforementioned nuisance warning problem, such efforts have centered on modifications of the warning envelope and modifications of the signal processing circuitry, such as modifications to rate limiters and filters designed to prevent the system from generating false warnings on approaches to particular airports. However, no satisfactory solution has been found because such modifications tend to reduce the warning time provided by the system in potentially dangerous situations.